Direction of gas flow in a central air-conditioning unit

I've been trying to figure out why our outdoor condenser unit is rattling sometimes, especially when it is cooler outside. On a hot day it is nice and quiet, but on a cool day it rattles. Why is that ?

My theory is that I made a mistake earlier this summer by wrapping the cold gas pipe into an insulator outside. I was making an assumption that the condenser unit is pumping cold gas into the house. Turns out, it is the opposite -- it pumps hot gas into the house, which then passes through an expansion device (aka A-coil in the furnace), absorbs energy, becomes cold, and reenters the outdoor condenser unit.

By the time it gets to the condenser unit it should not be as cold (i.e. low-pressure) as it was when it just expanded in the A-coil. The lower the pressure of the gas the more work the compressor needs to do to turn it back into hot high-pressure gas. Hence, my theory -- the compressor couldn't keep up and would start rattling. That is why on a hot day it is quieter, since the gas gets warmed up by the environment by the time it gets to the condenser.

So, I unwrapped the insulator from the cold gas pipe outside. This way it will get somewhat warmed up by the environment by the time it gets to the condenser. I will be watching and listenning to it over the next few days and if that didn't help I might have to call the HVAC dude.